The present invention relates to lubricant-coated solder spheres suitable for use in the formation of solder bumps on a substrate for connecting electronic components to the substrate. The invention also relates to a method for coating solder spheres with a lubricant.
As electronic devices have become increasingly light, thin, short, or compact, electronic components used in these devices have been made compact and multi-functional. In order to produce a multi-functional electronic component, it is necessary to assemble a plurality of IC""s in a single component and form a great number of leads extending from the IC""s.
Examples of such multi-functional electronic components include QFP""s (Quad Flat Packages) and SOP""s (Small Outline Packages), which have leads projecting from all four sides or from two opposing sides, respectively, of a square or rectangular body. These electronic components, however, are limited in the number of leads that can be formed thereon since the leads project from the sides of the component body which have limited areas. With a QFP or SOP, the number of leads is normally at most on the order of 200, no matter how narrow the distance between leads is.
More recently-developed electronic components are BGA""s (Ball Grid Array packages) and CSP""s (Chip Size Packages). These electronic components comprise a substrate having IC""s placed on the top surface thereof. On the bottom or back surface of the substrate, leads are formed in the shape of dots and are connected to the IC""s by passing through the substrate. The BGA""s and CSP""s (hereinafter collectively referred to as BGA""s) can be provided with a greater number of leads than QFP""s and SOP""s, thereby enhancing the multi-functionality of the components, since the leads are formed in the shape of dots on a wider-area surface of the substrate.
BGA""s are normally mounted on a printed circuit board by solder bumps which have been formed on the individual leads of the BGA""s. For this purpose, a BGA is positioned on a printed circuit board with the solder bumps formed on the BGA in alignment and in contact with the mating pads (also called lands or electrodes) of the printed circuit board, and the resulting assembly is heated so as to melt the solder bumps for soldering. Solder spheres (also called solder balls) are used to form these solder bumps on the back surface of the substrate of a BGA.
At present, solder spheres having a diameter of 0.76 mm are most common for use in BGA""s, but much smaller solder spheres having a diameter of 0.15 mm or 0.1 mm, for example, have begun to be used.
Solder spheres manufactured by a solder manufacturer are packed in a glass or plastic container and transported to a user. The solder spheres are used to form solder bumps on a BGA substrate by the user in the following manner. First, an adherent soldering flux is applied to the back surface of the BGA substrate such that only the dot-shaped leads formed on that surface are coated with the flux. The solder spheres are then placed on the flux-coated areas of the substrate by a solder sphere mounter such that each flux-coated lead receives a solder sphere and the spheres remain in that position by the adhesive properties of the flux. The solder sphere mounter comprises a suction head having suction ports with the same pattern as that of the leads formed on the BGA substrate on which solder bumps are to be formed. Each suction port of the suction head of the mounter can hold a solder sphere by the action of suction. After the mounter is positioned such that the suction ports of the mounter are in alignment with the leads of the BGA substrate, the solder spheres held by the suction ports are released for placement on the flux-coated BGA substrate. Thereafter, the BGA substrate having solder spheres on the flux-coated leads formed on its surface is heated in a heating device such as a reflow furnace such that the solder spheres are flattened by melting to form solder bumps bonded to the substrate by soldering.
The above-described technique for the formation of solder bumps on a BGA substrate to assemble a multi-functional electronic component involves the following problems: incomplete adhesion of the solder spheres on the flux-coated leads leading to a loss in bonding strength, grime or contamination appearing on the substrate surface in the vicinity of the leads on which solder bumps are to be formed, and failure to release all or part of the solder spheres from the solder sphere mounter.
The present inventors investigated these problems and found that incomplete adhesion of solder spheres and the formation of grime or contamination on the substrate were due to blackening of the surfaces of the solder spheres by deposition of black powder thereon. With such solder spheres having a black appearance, the black powder deposited on the surfaces of the spheres may interfere with soldering, thereby preventing the solder spheres from completely adhering to the leads, or even if the blackened solder spheres can adhere to the leads, the black powder may drop off from the spheres and be deposited on the substrate surface in the vicinity of the leads to produce grime.
Freshly prepared solder spheres have clean surfaces. However, their surfaces may be blackened while they are transported to a user while packed in a container. If a container packed with fresh solder spheres is allowed to stand, the spheres do not exhibit blackening even after standing for a prolonged period. However, after transportation of such a container by a vehicle or airplane, the solder spheres in the container may assume a blackened appearance.
Such blackening of solder spheres may occur when users handle the solder spheres to place the spheres on a BGA substrate using a solder sphere mounter. This is because in order to pick up solder spheres by s suction head of the solder sphere mounter, the mounter may utilize vibration of a large number of solder spheres in a container, flotation of solder spheres by a compressed gas, or rotation of the container or the suction head while they are in intimate contact with each other.
Thus, blackening of solder spheres can take place during transportation of the spheres or placement thereof onto a BGA substrate. The mechanism of blackening is thought to be as follows. Shaking, swinging, vibration, rotation or similar movement of a container receiving solder spheres during transport or placement causes the solder spheres to rub against one another or against the container wall, which in turn causes the surfaces of the solder spheres to be scraped off or otherwise injured, resulting in the formation of powder of the solder as scrapings. Due to the large surface area of the resulting solder powder, the powder is readily oxidized and turns black. Furthermore, the scraped solder spheres have new active surfaces, which are also readily oxidized, thereby providing the solder spheres with a blackened appearance. Blackening of the solder spheres becomes more conspicuous if the scraped-off black powder is deposited on the surfaces of the solder spheres.
Failure to release solder spheres from the suction ports of a solder sphere mounter may be caused by the mechanism that the solder spheres are secured in the suction ports of the mounter by biting into the ports, which interferes with easy release of the spheres from the suction ports.
Solder spheres consist of soft metals such as lead and tin, and hence their surfaces have low slip properties. Therefore, shaking, swinging, or vibrating of a large number of solder spheres in a container may cause those solder spheres which are in contact with the container wall or with one another to be rubbed without good slippage, thereby resulting in scraping of the surfaces of the solder spheres, which causes blackening of the solder spheres. The soft solder spheres tend to be bitten by the edges of the suction ports of the mounter and such solder spheres do not readily fall down from the suction ports by application of a moderate shock or reverse injection of air to the suction head when they are to be released from the mounter.
It has been found that improvement in slip properties of the solder spheres can eliminate or alleviate these problems. Thus, solder spheres having improved slip properties can well withstand injuring or surface scraping when vibrated, swung, shaken or otherwise moved in a container. In addition, they can fall down more easily from the suction ports when they are to be released from the mounter.
Thus, the present invention provides solder spheres having a substantially uniform coating of a lubricant on the surfaces thereof to improve the slip properties of the solder spheres.
In another aspect, the present invention provides a method for producing solder spheres coated with a lubricant, which comprises dipping solder spheres in a solution of a lubricant dissolved in an organic solvent with a concentration of from 10 ppm to 1000 ppm, removing the solder spheres from the solution, and volatilizing the solvent remaining on the solder spheres, leaving lubricant-coated solder spheres.
The present invention also provides a substrate for mounting an electronic component which has solder bumps formed from the above-described lubricant-coated solder spheres on the surface thereof.
The solder bumps can be formed by placing the luricant-coated solder spheres on the surface of a substrate and heating the substrate to cause the solder spheres placed thereon to melt and form solder bumps bonded to the substrate. Preferably, the substrate used for placement has a surface coating of a soldering flux in those areas on which the solder spheres are to be placed.